The long-term objectives of the project are the identification of morphological and biochemical diversity within the efferent innervation and the patterns of the associated synaptic connectivity in the developing, as compared to adult, mouse cochlea. The P.I. proposes to study the complex synaptic connectivity of the GABAergic and cholinergic efferent fibers of the inner spiral bundle. The research goals aim to answer two principle questions 1) Is the CNS type of synaptic connectivity the P.I. has described in the developing mouse a transitory developmental event, or is it also present, but not noticed before, in the adult cochlea? 2) Are the synaptic arrays of compound synapses formed only by GABAergic fibers, or are these patterns also used by other efferent subsystems? In order to answer these questions, the P.I. proposes the following projects: 1) Ultrastructural studies, using conventional electron microscopy, of the synaptic connectivity of efferent fibers in the adult mouse cochlea. The study will elucidate: a) The patterns of connectivity: The P.I. will identify different synaptic connections (14 of which were demonstrated in the developing mouse) formed between the efferents and afferents, and between the afferents themselves; b) The types of synapses: The P.I. will try to detect if the adult organ of Corti uses compound synapses as discovered in the developing (up to 1 PN) mouse, i.e., triadic, serial and converging synapses so far identified only in the organ of Corti in culture; c) The synaptic appendages used by the inner hair cells and by both the afferent and efferent terminals in reaching distant targets. 2 Immunocytochemical/ultrastructural studies to investigate GABAergic and cholinergic efferent fibers in the maturing and adult cochlea that may partake in compound synapses at the lower level of the inner hair cell receptor poles will also be conducted. Additionally, the P.I. plans to identify ultrastructurally the morphological and immunochemical character of vesiculated endings that express GAP-43 in the inner spiral bundle, the only neuronal pathway that contains this growth protein in the adult cochlea.